After parturition, the human breast produces colostrum, which is enriched with antimicrobial and antiinflammatory proteins. In the absence of placental progesterone, normal breast milk production occurs within a few days. The lobuloalveolar structures produce milk, which is subsequently modified by the ductal epithelium. Lactogenesis and maintenance of milk production (galactopoiesis) require stimulation by pituitary PRL in the presence of normal levels of other hormones, including insulin, cortisol, and thyroid hormone. Although placental estrogen stimulates PRL secretion during pregnancy, the stimulus for PRL secretion during the nursing period is suckling by the infant (
Fig. 44.38 ). Levels of PRL are directly correlated with frequency and duration of suckling at the nipple. The link between suckling at the nipple and PRL secretion involves a neuroendocrine reflex in which dopamine secretion at the median eminence is inhibited (the PRL release inhibitory factor; see ). It is also possible suckling increases secretion of unidentified PRL-releasing hormones.
PRL also inhibits release of GnRH, and consequently, nursing can be associated with lactational amenorrhea (see
Fig. 44.38 ). This effect of prolactin has been called “nature’s contraceptive,” and it may play a role in spacing out pregnancies. However, only regular nursing over a 24-hour period is sufficient to induce a PRL-induced anovulatory state in the mother. Thus lactational amenorrhea is not an effective or reliable form of birth control for most women. Inhibition of GnRH by high levels of PRL is important clinically. A prolactinoma is the most common form of hormone-secreting pituitary tumor, and hyperprolactinemia is a significant cause of infertility in both sexes. Hyperprolactinemia can likewise be associated with galactorrhea (inappropriate flow of breast milk) in men and women.
Suckling at the nipple also stimulates release of oxytocin from the pars nervosa (see
Fig. 44.38). Contraction of myoepithelial cells induces milk let-down, or expulsion of milk from the
Invasive breast cancer (IBC) isamajorcancerinwomenandcanbeclassifiedintoseveralcategories.MostnewlydiagnosedIBCisclassifiedasluminal A, whichisusuallyderivedfromluminalcellsofterminalductsoralveoli.LuminalAIBCdisplayssomeepithelialorganization,includingEcadherin–mediatedcell-cellcontacts,andispoorlymotileandpoorlyaggressive.Thisformalsoexpressestheestrogen receptor α (ERα) andisdependentonestrogenicstimulationforgrowth.EarlydiagnosisofluminalAIBCconfersagoodprognosis.Treatmentforearlysmalltumorsthatare“node-negative”(i.e.,havenotspreadtonearbylymphnodes)typicallyinvolvessurgicalremoval(“lumpectomy”),followedbyradiationtreatment,followedby5yearsofdailytamoxifentreatment.TamoxifenisaSERMthatopposesestrogeninthebreast.